The immigration reform bill before the Senate is called the Border Security, Economic Opportunity and Immigration Modernization Act of 2013. The order is no accident. Border security is the linchpin, and few Republicans will support the broader legislation unless they are convinced the border can be secured and that the United States will not see another surge in illegal immigration as it did following the 1986 reform bill.

But how can Congress know whether the borders are secure? Despite an enormous buildup of Border Patrol agents, fencing and technology over the past two decades, the U.S. government has yet to assess whether these expenditures have actually been effective in reducing illegal immigration. In a new report for the Council on Foreign Relations titled “Managing Illegal Immigration to the United States: How Effective is Enforcement?” we argue that the administration can gain congressional and public trust only by developing and publicly reporting real measures of the effectiveness of border enforcement. Such accountability, coupled with better congressional oversight, would help reassure a skeptical public that the U.S. government is indeed serious about controlling illegal migration.

The Department of Homeland Security has not risen to the challenge. Senior officials were recently called out by Republicans and Democrats on the House Homeland Security Committee for failing to provide any useful metrics to Congress on border security. The Border Condition Index, long promised by DHS, is nowhere in sight. The administration has instead relied on a single figure — the total number of apprehensions in the border region — and has made its case by arguing that the big decline since 2006 shows that migrants have been deterred from coming illegally. But with the uptick in apprehensions, which have grown more than 10 percent this fiscal year, that number is less helpful to DHS in making its case that the border is more secure than ever.

What should a more comprehensive assessment include? As the CompStat revolution in New York City policing has shown, the numbers that matter for law enforcement are the rates at which the laws under their jurisdiction are broken. For border enforcement, that means how many people are still entering the United States illegally, and what percentage of those who try are stopped by law enforcement. Astonishingly, DHS does not try to report either number.

In the paper, we do our best to establish those numbers using outside data and the limited data DHS has publicly released. Analysis based on surveys of Mexican migrants carried out for many years by academic organizations finds that the apprehension rate for illegal border crossers is 50 percent to 60 percent, roughly double the rate of the mid-1990s when the border buildup began. Estimates based on “recidivism,” in which the Border Patrol tracks arrests of repeated crossers, also suggest steady improvements and an apprehension rate in the 40 percent to 55 percent range. All the methods show a big drop in the number of successful illegal entries each year, from close to 1.5 million a decade ago to less than 500,000 in recent years.

Interestingly, a 50 percent apprehension rate is in line with a recent test conducted by the new airborne Vader radar system now in use by DHS at the border. The test reportedly showed during a three-month period over the Sonoran Desert last winter that Border Patrol agents apprehended 1,874 people but missed 1,962 others. DHS has not confirmed or denied those press reports.

The Senate immigration proposal for the first time actually sets a border performance target for DHS. It calls for a 90 percent “effectiveness rate” in high-traffic sectors of the border, a target that Border Patrol Chief Michael Fisher has said is within reach. The number is based on estimates the Border Patrol has been collecting for more than a decade that were shared last year for the first time with the Government Accountability Office, an encouraging development. The Senate target requires that Border Patrol arrest or “turn back” to Mexico 90 percent of illegal crossers.

While the target is feasible, the methodology that underlies it does not tell the whole story. The numbers are based on Border Patrol observations of physical evidence of illegal crossings and thus certainly undercount the numbers who sneak across the border without leaving any trace. Legislation recently approved by the House Homeland Security Committee would require a far more comprehensive set of measures from DHS.

While this more comprehensive set may produce numbers short of 90 percent, it would finally ground the debate over border security in facts rather than wishful thinking. That should help in building both public and congressional confidence that a reasonable level of border security is genuinely achievable and that the government is trying to measure and report it accurately.

Edward Alden is a senior fellow at the Council on Foreign Relations; Bryan Roberts is an economist at Econometrica; and John Whitley is adjunct lecturer at The George Washington University. Roberts and Whitley previously served in senior positions in program analysis and evaluation at the Department of Homeland Security.